1. Field of the Invention
The present invention relates generally to a time base corrector, and more specifically to a time base corrector incorporated in a so-called timeplex video tape recorder in which component signals (color difference signals) are recorded in time division multiplex (timeplex) method. In the timeplex method, a signal is time-divided into plural pulses and the divided signal is reproduced into the original signal. Further, since pulses of another signal can be inserted within the quiescent time, it is possible to transmit plural signals through a multiplex transmission line.
2. Description of the Prior Art
In general, in a time base corrector incorporated in a video tape recorder, a clock signal is generated in synchronization with a horizontal synchronizing signal and a burst signal extracted from a reproduced signal; component signals (color difference signals such as R-Y signal and B-Y signal) included in the reproduced video signal in accordance with timeplex method are converted from analog to digital in response to the generated clock signal; the converted digital signals are written into a memory unit by using the above clock signal as a write clock signal; the written data signals are read out and converted from digital to analog in response to a stable reference clock signal in order to obtain a video signal; and the time base of the video signal is corrected in extending the time base of each component signal before the digital-to-analog conversion.
By the way, in the case where there exists a difference in the relative velocity between a tape head and tape between recording and reproducing processes, hue fluctuations often remain. This phenomenon is called velocity error. The correction of velocity error is to correct the phenomenon such that a clock signal generated in the time base corrector becomes discontinuous in phase when velocity fluctuations having a frequency higher than that of the horizontal synchronizing signal occur in the reproduced video signal within each horizontal period segment of the reproduced video signal.
In the conventional time base corrector, first the phase error generated during each horizontal period segment is detected and then the clock signal is phase modulated on the basis of a rectilinearly inclined signal or a curvedly inclined signal, for instance, so as to eliminate phase error when reading out the video data for each horizontal period segment from the memory unit. The above-mentioned method is also called velocity error correction, and is disclosed in great detail in U.S. Pat. No. 4,321,619. In the above method, since the phase error generated within each horizontal period segment of the reproduced video signal is gradually corrected so as to vary along the rectilinearly inclined signal or curvedly inclined signal, it is possible to correct the velocity error generated in the reproduced video signal without accumulating or integrating the velocity error.
However, the above conventional velocity error correction method cannot perfectly correct the velocity error in the case where the video signal recorded on the basis of the timeplex method is reproduced. This is because the time base of the reproduced video signal is continuously corrected according to the velocity error generated during each horizontal period segment in spite of the fact that the component signals are separately timeplexed within each horizontal period segment. In other words, the conventional method has no consideration for the fact that the component signal timeplexed at the second half of each horizontal period segment has an initial offset velocity error.
The shortcomings of the conventional velocity error correction method will be described in more detail hereinafter with reference to the attached drawings under Detailed Description of the Preferred Embodiments.